With the recent wide spread use of portable electronic devices, such as mobile phones and notebook personal computers, there has been a strong demand to develop small, light nonaqueous electrolyte secondary batteries having high energy density. Among such secondary batteries are lithium-ion secondary batteries. A lithium metal or lithium alloy, a metal oxide, carbon, or the like is used as the material of the negative electrode of a lithium-ion secondary battery. These materials can desorb and absorb lithium.
At present, lithium-ion secondary batteries are actively being researched and developed. Among others, a lithium-ion secondary battery using, as a positive electrode material, a lithium-transition metal composite oxide, particularly, a lithium-cobalt composite oxide (LiCoO2), which is relatively easily synthesized, supplies a 4V-level high voltage. For this reason, such lithium-ion secondary batteries are being commercialized as batteries having high energy density. Many lithium-ion secondary batteries using a lithium-cobalt composite oxide (LiCoO2) have been developed so far to obtain excellent initial capacity characteristics or cycle characteristics, and various fruits have already been produced.
However, a lithium-cobalt composite oxide (LiCoO2) uses a rare, expensive cobalt compound as a raw material and therefore causes an increase in battery cost. For this reason, it is preferred to use a material other than a lithium-cobalt composite oxide (LiCoO2) as a positive electrode active material.
Recently, there has been increased the demand to use lithium-ion secondary batteries not only as small secondary batteries for portable electronic devices but also as large secondary batteries for power storage or those for electric vehicles and the like. It is expected that if the cost of active materials is reduced so that cheaper lithium-ion secondary batteries can be produced, a significant ripple effect will reach a wide variety of fields. Among lithium-transition metal composite oxides newly proposed as positive electrode active materials for lithium-ion secondary batteries are a lithium-manganese composite oxide (LiMn2O4) using manganese, which is cheaper than cobalt, and a lithium-nickel composite oxide (LiNiO2) using nickel.
The material of a lithium-manganese composite oxide (LiMn2O4) is cheap. Further, a lithium-manganese composite oxide has safety about thermal stability, in particular, excellent safety about ignition and the like and therefore can be said to be a promising alternative material to a lithium-cobalt composite oxide (LiCoO2). However, a lithium-manganese composite oxide has a theoretical capacity which is only about half that of a lithium-cobalt composite oxide (LiCoO2) and therefore is disadvantageously difficult to meet the demand to increase the capacity of lithium-ion secondary batteries, which has been raised year by year. Another disadvantage of a lithium-manganese composite oxide is that at 45° C. or more, its self-discharge is significant and therefore reduces the charge/discharge life.
On the other hand, a lithium-nickel composite oxide (LiNiO2) has approximately the same theoretical capacity as a lithium-cobalt composite oxide (LiCoO2) and supplies a somewhat lower battery voltage than a lithium-cobalt composite oxide. For this reason, a lithium-nickel composite oxide is less likely to be decomposed due to the oxidation of an electrolyte solution and is expected to have a higher capacity and therefore is actively being developed. However, a lithium-ion secondary battery produced using a lithium-nickel composite oxide purely formed of nickel alone as a positive electrode active material without replacing nickel with another element has lower cycle characteristics than a lithium-cobalt composite oxide. Further, when such a lithium-ion secondary battery is used or stored in a high-temperature environment, it disadvantageously tends to impair battery performance. Another disadvantage of a lithium-nickel composite oxide is that when it is left alone in a high-temperature environment with the battery fully charged, it releases oxygen at lower temperature compared to a cobalt-based composite oxide.
To eliminate these disadvantages, there has been considered the addition of niobium, which is an element having a higher valence than nickel, to a lithium-nickel composite oxide. For example, Patent Literature 1 proposes the following lithium-transition metal composite oxide in order to improve thermal stability when a short-circuit occurs in a positive electrode active material: the lithium transition metal composite oxide consists of particles having a composition including at least two or more compounds consisting of lithium, nickel, cobalt, element M, niobium, and oxygen represented by LiaNi1−x−y−zCOxMyNbzOb where M is one or more elements consisting of Mn, Fe, and Al; 1.0≤a≤1.1; 0.1≤x≤0.3; 0≤y≤0.1; 0.01≤z≤0.05; and 2≤b≤2.2; the particles are approximately spherical, and an approximately spherical shell layer containing at least one or more compounds having a higher niobium concentration than the above composition are present near or inside the surfaces of the particles; and a and β simultaneously satisfy conditions 80≤α≤150 and 0.15≤β≤0.20, respectively, where α represents the discharge capacity [mAh/g] in a range of 2 V to 1.5 V indicated by the potential of the positive electrode at the initial discharge; and β represents the half-width [deg] of the (003) surface of the layered crystal structure in X-ray diffraction.
Patent Literature 2 proposes the following lithium-transition metal composite oxide in order to improve the thermal stability of a positive electrode active material and to increase the charge/discharge capacity: the lithium-transition metal composite oxide is represented by Li1+zNi1−x−yCoxNbyOz where 0.10≤x≤0.21; 0.01≤y≤0.08; and −0.05≤z≤0.10; and the standard deviation of the intensity ratio of the peak intensity INb of the L line of Nb to the peak intensity INi of the L line of Ni measured by an energy dispersive method is within ½ of the average value of the intensity ratio INb/INi.
Patent Literature 3 proposes the following lithium-transition metal composite oxide in order to obtain a positive electrode active material that has a large capacity and has higher thermal stability at the time of charge: the lithium-transition metal composite oxide is represented by a composition formula LixNiaMnbCocM1dM2eO2 where M1 is at least one or more elements selected from the group consisting of Al, Ti, and Mg; M2 is at least one or more elements selected from the group consisting of Mo, W, and Nb; 0.2≤x≤1.2; 0.6≤a≤0.8; 0.05≤b≤0.3; 0.05≤c≤0.3; 0.02≤d≤0.04; 0.02≤e≤0.06; and a+b+c+d+e=1.0.
Patent Literature 4 proposes the following lithium-transition metal composite oxide in order to achieve both the charge/discharge capacity characteristics and safety of a lithium-ion secondary battery and to suppress the degradation of cycle characteristics: the lithium-transition metal composite oxide has a structure in which a lithium composite oxide represented by LixNi(1−y−z−a)CoyMnzMaO2 where M is at least one element selected from the group consisting of Fe, V, Cr, Ti, Mg, Al, Ca, Nb, and Zr; and x, y, and z are 1.0≤x≤1.10, 0.45≤y+z≤0.7, and 0.2≤z≤0.5; and 0≤a≤0.02 is coated with a substance A which is a compound consisting of at least one element selected from the group consisting of Ti, Sn, Mg, Zr, Al, Nb, and Zn.
Patent Literature 5 proposes the following lithium-transition metal composite oxide in order to obtain an positive electrode active material having excellent thermal stability and a high charge/discharge capacity: the lithium-transition metal composite oxide is represented by Li1+zNi1−x−yCoxMyO2 where x, y, z satisfy conditions 0.10≤x≤0.21, 0.015≤y≤0.08, and −0.05≤z≤0.10; and M consists of at least two elements selected from the group consisting of Al, Mn, Nb, and Mo, which have higher affinity with oxygen than nickel, and the average valence number of M exceeds 3; and impregnation with, or adherence of, two M is performed in the production process.
The demand to increase the capacity of small secondary batteries for portable electronic devices and the like has been increased year by year. The trend to use lithium-ion secondary batteries as large secondary batteries has grown. Also, there is a strong demand to use lithium-ion secondary batteries as power supplies for hybrid vehicles and electric vehicles or as stationary storage batteries for power storage. Further, these batteries are required to increase life, and it is important that these batteries have excellent cycle characteristics. For such uses, positive electrode active materials are required to have a higher charge/discharge capacity and to further improve thermal stability and cycle characteristics.